Implement auto-conversion from xcf to png

Snakefile

@@ -360,6 +360,11 @@ rule png_rotate:
             raise ValueError("Cannot double-rotate")
         shell('convert {input:q} -rotate {wildcards.angle:q} {output:q}')
 
+rule xcf_to_png:
+    input: 'graphics/{filename}.xcf'
+    output: 'graphics/{filename}.png'
+    shell: 'convert {input:q} -flatten {output:q}'
+
 rule R_to_html:
     '''Render an R script as syntax-hilighted HTML.'''
     input: '{dirname}/{basename}.R'